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Detective Inspector Gary Jubelin said there was still hope the boy, who vanished from his grandmother's NSW mid-north coast home on September 12 last year, was still alive.

He said police were investigating a number of possibilities, including that William may have been abducted by a pedophile ring.

"We're talking about human intervention," he said.

William Tyrrell should be enjoying his fourth birthday today. (Supplied) ()

"If you do have information, there is no excuse for not coming forward."

It is hoped Braveheart's "Where's William" campaign will help extend the reach of the search.

William turns four today but his family will be unable to celebrate, instead gathering with friends to bake a cake in his name and share memories of the little boy who disappeared on the NSW mid-north coast nearly 300 days ago.

In a heartbreaking letter released earlier this week to the Daily Telegraph, his family spoke about what they would be doing if their son was home.

"If William were home, he would be beside himself with excitement and making a long list of presents he would like which would definitely include toys like superhero action figures and dress-ups," the letter read.

William Tyrrell's parents pictured in a NSW Police interview, and inset, the missing toddler. (Supplied) (Supplied)

"If William were home, he’d be making plans to celebrate his special day and a party would most definitely be on his agenda."

William went missing from his grandmother's home in Kendall on September 12.

Police fear he has been abducted, saying it was possible he was being held by a pedophile ring.

His family said they would spend William's birthday thinking about their missing boy.

Anybody with any information is urged to call CrimeStoppers on 1800 333 000.